Conventionally, as a material for bearings used in various fields such as a car and various industrial machines, there has been used a high carbon chromium bearing steel such as SUJ2 defined according to JIS G 4805 (1999).
The present applicant has submitted various proposals in order to obtain those more excellent in characteristics as the bearing steels. For example, in Patent Literature 1, in order to make favorable the polishability in working into a product shape, and to obtain a favorable rolling contact fatigue life with stability, particularly, the size and the density of Al type nitrogen compounds dispersed in steel, and the size and the area ratio of cementite are defined. Whereas, in Patent Literature 2, in order to further improve the rolling contact fatigue life, the ratio of carbide area ratios respectively observed in the Cr concentrated part due to stripe segregation and the Cr non-segregated part is defined. Further, Patent Literature 3 submits the following proposal: in order to obtain a bearing steel wire rod suitable for deep wire drawing, which is not broken even when subjected to deep wire drawing so that the wire drawing reduction ratio exceeds about 50% (further, 70%), the manufacturing conditions are controlled, thereby to set the mean circle equivalent diameter and the standard deviation of cementite after spheroidizing annealing equal to or smaller than given values for suppressing variations.
Incidentally, a bearing component can be obtained in the following manner: a bearing steel in a shape of wire rod, steel rod, or the like is subjected to cold working such as cutting, forging, or machining, thereby to be worked into a final shape. However, the wire rod or rod material as-hot rolled is too hard, and hence the cold working is difficult to perform. For this reason, in order to improve the cold workability, generally, spheroidizing annealing is carried out before cold working. In order to ensure excellent cold workability, it is important to control the microstructure of the steel after the spheroidizing annealing (spheroidizing annealed material). Thus, various proposals have been submitted.
For example, Patent Literature 4 submits the following proposal: by controlling the hot rolling temperature and the cooling rate in the manufacturing steps, pro-eutectoid network cementite is reduced, and the pearlite lamellar spacing is coarsened. However, with this technology, spheroidal cementite is dispersed uniformly and finely. For this reason, it is considered difficult to say that the hardness is sufficiently reduced.
Whereas, Patent Literature 5 discloses a patent in which, after a spheroidizing annealing treatment, the ferrite mean particle size and the cementite mean particle size after cold wire drawing are defined, thereby to improve the cold workability. However, the C and Cr contents may be high, resulting in the formation of eutectic carbide. For this reason, diffusion annealing becomes essential. Furthermore, after spheroidizing annealing, cold wire drawing is carried out at 20 to 40%, resulting in the degradation of yield of the steel. This conceivably results in a higher manufacturing cost.
However, in recent years, from the viewpoints of further cost reduction and CO2 reduction, more excellent cold workability has been demanded.